Soft magnetic materials are used for applications, such as core materials in inductors, stators and rotors for electrical machines, actuators, sensors and transformer cores. Traditionally, soft magnetic cores, such as rotors and stators in electric machines, are made of stacked steel laminates. Soft Magnetic Composite, SMC, materials are based on soft magnetic particles, usually iron-based, with an electrically insulating coating on each particle. By compacting the insulated particles optionally together with lubricants and/or binders using the traditionally powder metallurgy process, the SMC parts are obtained. By using this powder metallurgical technique it is possible to produce materials giving a higher degree of freedom in the design of the SMC component than by using the steel laminates as the SMC material can carry a three dimensional magnetic flux and as three dimensional shapes can be obtained by the compaction process.
Two key characteristics of an iron core component are its magnetic permeability and core loss characteristics. The magnetic permeability of a material is an indication of its ability to become magnetised or its ability to carry a magnetic flux. Permeability is defined as the ratio of the induced magnetic flux to the magnetising force or field intensity. When a magnetic material is exposed to a alternating magnetic field, energy losses, core losses, occur due to both hysteresis losses and eddy current losses. The hysteresis loss is brought about by the necessary expenditure of energy to overcome the retained magnetic forces within the iron core component and is proportional to the frequency of the alternating field. The eddy current loss is brought about by the production of electric currents in the iron core component due to the changing flux caused by alternating current (AC) conditions and is proportional to the square of the frequency of the alternating field. A high electrical resistivity is then desirable in order to minimise the eddy currents and is of especial importance at higher frequencies. In order to decrease the hysteresis losses and to increase the magnetic permeability of a core component for AC applications it is generally desired to heat-treat the compacted part.
Research in the powder-metallurgical manufacture of magnetic core components using coated iron-based powders has been directed to the development of iron powder compositions that enhance certain physical and magnetic properties without detrimentally affecting other properties of the final component. Desired component properties include e.g. a high permeability through an extended frequency range, low core losses, high saturation induction, (high density) and high strength. Normally an increased density of the component enhances all of these properties.
The desired powder properties include suitability for compression moulding techniques, which i.a. means that the powder can be easily moulded into a high density, high strength component which can be easily ejected from the moulding equipment and that the components have smooth surface finish.
The present invention concerns a new powder composition having the desired powder properties as well as the use of the powder composition for the preparation of soft magnetic composite components. The new composition can be compacted (and heat treated) to components having the desired properties.
The present invention also concerns a method for manufacturing soft magnetic iron-based components having excellent component properties as well as the soft magnetic component per se.